Since the traffic volume in the backbone networks has been rapidly increasing due to the growth of the Internet, an ultra-high-speed such as 100 Gbps and long-haul optical communication system has been desired. As a technology to realize such an ultra-high-speed and long-haul optical communication system, an optical phase modulation system and a polarization multiplexing/demultiplexing technology which utilize a digital signal processing technology have been attracting attention.
The optical phase modulation system is not a system to perform data modulation on the light intensity of a transmitting laser beam just like a conventional optical intensity modulation system, but a system to perform data modulation on the optical phase of a transmitting laser beam. Well-known optical phase modulation systems include a binary phase shift keying (BPSK) system, a quadrature phase shift keying (QPSK) system, an 8-phase shift keying (8PSK) system, a quadrature amplitude modulation (QAM) system, and the like.
If the BPSK method is employed, one bit (for example, “0” and “1”) is allocated to two kinds of optical phases (for example, 90 degrees and 270 degrees). FIG. 7 illustrates an example of a constellation diagram for optical signals having modulated by the BPSK system. If the QPSK method is employed, two bits (for example, “00”, “01”, “11”, and “10”) are allocated to four kinds of optical phases (for example, 45 degrees, 135 degrees, 225 degrees, and 315 degrees) respectively. In this case, since two bits can be allocated to a single kind of the optical phase, it is possible to reduce the symbol rate in the QPSK system to one half of the symbol rate in the optical intensity modulation system (that is, a bit rate).
As mentioned above, it is possible in the multilevel optical phase modulation system to reduce a symbol rate (baud rate) by allocating a plurality of bits to a single symbol. Since this enables the operation speed of each electric device to decrease, it can be expected to reduce the manufacturing cost of a communication device. If the BPSK system is employed, it is impossible to obtain the effect of reducing a symbol rate because a single bit is allocated to only one type of optical phase. Because of a large distance between symbols, however, it is possible to obtain a large tolerance for phase noise due to the spontaneous emission light in an optical amplifier and a nonlinear optical effect. The BPSK system, therefore, is suitable for the ultra-long-haul optical transmission such as an intercontinental transmission.
In order to receive the signal light modulated by optical phase modulation, an optical coherent system is employed. In the optical coherent system, the signal light and the laser light having the frequency almost identical to that of the signal light (called local oscillation light) are combined in an optical element called a 90-degree hybrid, the output of which is received by a photo detector. Here, for the sake of simplicity, it is assumed that the polarization state of the signal light and the local oscillation light is in the same linear polarization. If the optical coherent system is employed, the alternating-current component of electric signals output from the photo detector becomes beat signals of the signal light and the local oscillation light. The amplitude of the beat signal is proportional to the light intensity of the signal light and the local oscillation light. And the phase of the beat signal becomes the difference in the optical phase between the signal light and the local oscillation light if the carrier frequency of the signal light and the optical frequency of the local oscillation light are the same. Moreover, if the optical phase of the local oscillation light is the same as the optical phase of the laser light which is input into an optical modulator at a transmission end, the phase of the beat signal becomes equal to the optical phase which has been applied to the laser light at the transmission end. It is possible, therefore, to demodulate transmitting data by converting the phase of the beat signal into a bit sequence using a symbol mapping.
In an actual communication device, however, the value of the carrier frequency of the optical signal is not completely in accord with that of the optical frequency of the local oscillation light. Moreover, the optical phase of the local oscillation light in the optical receiver does not necessarily correspond to the optical phase of the laser light input into the optical modulator in the optical transmitter. It is necessary, therefore, to compensate the influence of an optical phase deviation of the difference in the optical phase between the signal light input into the optical modulator in the optical transmitter and the local oscillation light, and the influence of an optical carrier frequency deviation of the difference between the carrier frequency of the signal light and the optical frequency of the local oscillation light. It is possible to perform a process for compensating the optical phase deviation and the optical carrier frequency deviation by using a digital signal processing technology.
The polarization multiplexing/demultiplexing technology has been attracting attention as another technology to realize the ultra-high-speed optical communication system. In the polarization multiplexing/demultiplexing technology, two independent optical signals whose carrier waves are located in the same frequency band and polarization states are orthogonal to each other are multiplexed in the optical transmitter. And then the two independent optical signals are demultiplexed from received signals in the optical receiver. This makes it possible to realize a double transmission speed. Since a symbol rate (baud rate) of the optical signal becomes half adversely in such case, it is possible to reduce the operation speed of electric devices. The polarization multiplexing/demultiplexing technology, therefore, makes it possible to reduce the manufacturing cost of a communication device.
By combining the above-mentioned optical phase modulation system and the polarization multiplexing/demultiplexing technology, it is possible to realize an ultra-high-speed and long-haul optical communication system which can transmit signals at 100 Gbps. A technology has been proposed by which a process for compensating the optical carrier frequency deviation and the optical phase deviation, and a process for demultiplexing multiplexed signals into two independent optical signals (polarization demultiplexing processing) are performed by means of the digital signal processing technology, and the demodulation is performed with a high degree of accuracy. Such a method is called an optical digital coherent communication system (see patent literature 1, for example). It is possible to realize the above-mentioned digital signal processing by means of a digital signal processing circuit which is implemented in a large scale integration (LSI) or the like.
Next, the transmitting and receiving process in an ultra-high-speed optical communication system employing the optical digital coherent communication system will be described in detail. FIG. 4 illustrates a block diagram of a related optical transmitter 300 in a polarization multiplexed optical communication system employing the optical digital coherent communication system. A laser oscillator 310 emits a continuous light with a predetermined optical frequency. The continuous light is split into two continuous light beams in a polarization maintaining optical splitter 320, and input into two optical quadrature modulators 331 and 332, respectively. Driving signal generators 341 and 342 generate driving signals from transmission bit sequences. The optical quadrature modulators 331 and 332 perform phase modulation on the continuous light by the driving signals. A polarization multiplexer 360 multiplexes the output signal from the optical quadrature modulator 331 and the output signal from the optical quadrature modulator 332 in a state where the polarization states are orthogonal to each other, and outputs a multiplexed signal as polarization multiplexed transmitting light to an optical transmission line. The related optical transmitter 300 with above configuration is called an optical quadrature modulator and can be available for any optical phase modulation system. Without being limited to this configuration, it is also possible to employ a configuration of an optical transmitter which is specialized for each of optical phase modulation systems other than the optical quadrature modulation system.
FIG. 5 illustrates a block diagram of a related optical receiver 400 in the polarization multiplexed optical communication system employing the optical digital coherent communication system. The related optical receiver 400 receives a received optical signal through an optical transmission line. Local oscillation light with approximately the same optical frequency as the carrier frequency of the received optical signal is input into a 90-degree hybrid 410 together with the received optical signal. The 90-degree hybrid 410 demultiplexes the received optical signal into optical signal components, each of which has a polarization state parallel to each of two polarization axes orthogonal to each other, and outputs four optical signals in total composed of real part components and imaginary part components of the respective optical signal components. These four optical signals are converted by four optical detectors 421 to 424 into four analog electric signals, which are then converted into four digital electric signals by four analog-digital converters (ADC) 431 to 434. The digital electric signals output from the analog-digital converters (ADC) 431 to 434 are converted into digital electric signals which are sampled at a symbol rate of the received optical signal by a re-sampling unit (not illustrated), and then, they are input into a polarization demultiplexing processor 440. The polarization demultiplexing processor 440 extracts two independent polarization multiplexed optical signals on the basis of the four input digital electric signals. Optical carrier frequency deviation/optical phase deviation compensators 451 and 452 compensate an optical phase rotation in the extracted optical signals which is caused by the optical carrier frequency deviation and the optical phase deviation between the received optical signal and the local oscillation light. Finally, symbol decision units 461 and 462 demodulate them into an original transmission bit sequence.
According to the above-mentioned polarization multiplexed optical communication system employing the optical digital coherent communication system, it is possible to realize an ultra-high-speed and long-haul optical communication system which can transmit signals at 100 Gbps.